drm/i915/dsc: improve clarity of the pps reg read/write helpers

drm/i915/dsc: improve clarity of the pps reg read/write helpers

Make it clear what's the number of vdsc per pipe, and what's the number
of registers to grab. Have intel_dsc_get_pps_reg() return the registers
it knows even if the requested amount is bigger.

Cc: Suraj Kandpal <suraj.kandpal@intel.com>
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e2551b52ac0dd2b4ffe18d5e7733fafdc191d68a.1693933849.git.jani.nikula@intel.com